Solar cell with high-temperature front electrical contact, and its fabrication

ABSTRACT

A solar cell has an active semiconductor structure and a back electrical contact overlying and contacting an active semiconductor structure back side. A front electrical contact is applied overlying and contacting the active semiconductor structure front side. The front electrical contact has multiple layers including a titanium layer overlying and contacting the active semiconductor structure front side, a diffusion layer overlying and contacting the titanium layer, a barrier layer overlying and contacting the diffusion layer, and a joining layer overlying and contacting the barrier layer. The front electrical contact may be applied by sequentially vacuum depositing the titanium layer, the diffusion layer, the barrier layer, and the joining layer in a vacuum deposition apparatus in a single pumpdown from ambient pressure. A front electrical lead is affixed overlying and contacting an attachment pad region of the front electrical contact.

This invention relates to a solar cell and, more particularly, to asolar cell having a front electrical contact that is suitable for usefor extended periods of time and many duty cycles at elevatedtemperature.

BACKGROUND OF THE INVENTION

A solar cell has an active semiconductor structure including twosemiconductor layers in facing contact with each other at asemiconductor junction. When illuminated by the sun or otherwise, thesolar cell produces a voltage between the semiconductor layers andthence between a front side and a back side of the active semiconductorstructure. (“Front side” refers to the side facing toward the sun, and“back side” refers to the side facing away from the sun.) Advanced solarcells may include more than two semiconductor layers and theirrespective pairwise semiconductor junctions. The various pairs ofsemiconductor layers of the advanced solar cells are tuned to thevarious spectral components of the sun to maximize the power output ofthe solar cell.

Electrical contacts are applied to the front side and to the back sideof the solar cell. The back electrical contact normally is a continuouselectrically conductive layer deposited across all or most of the entireback side of the active semiconductor structure, inasmuch as the backside faces away from the sun during service. The front electricalcontact normally includes a plurality of interconnectedcurrent-gathering strips deposited upon the front side of the activesemiconductor structure. At discrete locations, attachment pad regionsare defined on the strips so that external electrical leads may beaffixed to the front electrical contact.

Solar cells are used in space and terrestrial applications. Particularlyfor space applications where the solar cells may be inaccessible formany years, and go through many thousands of sunlight/shade (i. e.,heating/cooling) duty cycles without any maintenance, the solar cellsmust be highly reliable. If the structure and performance of any elementof the solar cell degrade during service, the power output of that solarcell may be permanently reduced and eventually lost.

During service, the temperature of the solar cell is elevated above thetemperature of the ambient surroundings. Some advanced solar cells, suchas concentrator solar cells where multiples of the sun power areconcentrated on the face of the solar cell by a mirror, reachtemperatures of 80-140° C. in current designs. Over periods of 15-25years and thousands of duty cycles, diffusional processes may causeprogressive degradation of the structure of the solar cell thateventually leads to a decrease in performance.

Operable solar cells are known, but there is an ongoing need for anapproach to increase the reliability of existing types of solar cellsand to achieve high reliability in future types of solar cells. Thepresent invention fulfills this need, and further provides relatedadvantages.

SUMMARY OF THE INVENTION

The present invention provides a solar cell and a method for itsfabrication. The structure of the solar cell has improved reliability,as compared with conventional solar cells, during elevated temperatureexposure for extended periods of time and many duty cycles. The improvedreliability is based on an improved structure for the front electricalcontact. The front electrical contact may be vacuum deposited in asingle pumpdown from ambient pressure, reducing cost and the possibilityof contamination of the surfaces of the layers.

In accordance with the invention, a solar cell comprises an activesemiconductor structure having an active semiconductor structure frontside and an active semiconductor structure back side. The solar cellactive semiconductor structure may be of any operable type, but incurrent designs typically comprises a doped silicon layer and/or a dopedgallium arsenide layer. For the advanced solar cells with multiplesemiconductor layers, there are different pairs of active semiconductorlayers that are responsive to different wavelength components of thesolar spectrum. The solar cell active semiconductor structure produces avoltage between the active semiconductor structure front side and theactive semiconductor structure back side when illuminated. A backelectrical contact overlies and contacts the active semiconductorstructure back side. A front electrical contact overlies and contactsthe active semiconductor structure front side. The front electricalcontact includes the current collectors, the busbars, and the attachmentpads.

The front electrical contact has multiple layers comprising a titaniumlayer overlying and contacting the active semiconductor structure frontside, a diffusion layer overlying and contacting the titanium layer, abarrier layer overlying and contacting the diffusion layer, and ajoining layer overlying and contacting the barrier layer. The diffusionlayer is preferably gold, but other metals such as palladium may be usedin some circumstances. The barrier layer is preferably platinum, butnickel, palladium, rhodium, or other noble metals, may be used in somecircumstances. The joining layer is preferably silver, but other metalssuch as gold, aluminum, or copper may be used in some circumstances.

When the solar cell is interconnected with other solar cells to form anarray, a front electrical lead overlies and is affixed to an attachmentpad region of the front electrical contact. In some cases, the frontelectrical lead is a discrete wire, and in other cases the frontelectrical lead is deposited upon the front side of the solar cell. In atypical case, the front electrical lead is made of silver, kovar™,molybdenum, invar™, aluminum, or gold. As used here unless otherwisestated, the specification of a metal may include the unalloyed metal oralloys of the metal. In most cases, the pure metals are used to maximizeelectrical conductivity, but in some cases alloys of the specified metalwith other elements may be used for improved strength or otherproperties.

Preferably, the titanium layer has a thickness of from about 50Angstroms to about 250 Angstroms, the diffusion layer has a thickness offrom about 100 Angstroms to about 600 Angstroms, the barrier layer has athickness of from about 100 Angstroms to about 1000 Angstroms, and thejoining layer has a thickness of from about 20,000 Angstroms to about70,000 Angstroms.

A method for fabricating a solar cell comprises providing an activesemiconductor structure having an active semiconductor structure frontside and an active semiconductor structure back side, wherein the solarcell active semiconductor structure produces a voltage between theactive semiconductor structure front side and the active semiconductorstructure back side when illuminated, and a back electrical contactoverlying and contacting the active semiconductor structure back side. Afront electrical contact is applied overlying and contacting the activesemiconductor structure front side. The front electrical contact hasmultiple layers comprising a titanium layer overlying and contacting theactive semiconductor structure front side, a diffusion layer overlyingand contacting the titanium layer, a barrier layer overlying andcontacting the diffusion layer, and a joining layer overlying andcontacting the barrier layer. Other compatible features discussed hereinmay be used in relation to this fabrication approach.

The step of applying preferably includes the step of sequentiallydepositing the titanium layer, the diffusion layer, the barrier layer,and the joining layer. Most preferably, these layers are applied bysequentially vacuum depositing the titanium layer, the diffusion layer,the barrier layer, and the joining layer in a vacuum depositionapparatus in a single pumpdown from ambient pressure to the vacuumrequired for the deposition.

One source of degradation of a solar cell exposed to elevatedtemperatures during many duty cycles is a diffusion of metallic atomsfrom the front electrical contact to the bondline between the frontelectrical contact and the active semiconductor structure, and thenceinto the active semiconductor structure itself. The presence of foreignmetallic atoms at or near the bondline may degrade the adhesion betweenthe front electrical contact and the active semiconductor structure,causing it to separate from the front side of the active semiconductorstructure. Foreign metallic atoms diffusing into the activesemiconductor structure alter its doping, thereby potentially reducingits electrical performance.

In the present approach, the barrier layer is provided in the frontelectrical contact to serve as a diffusion barrier. Most other elementsdiffuse very slowly through such diffusion-barrier metals, so that thebarrier layer serves as a diffusion barrier against the diffusion of anyoverlying metallic elements into the underlying structure, andspecifically to the bondline between the front electrical contact andthe active semiconductor structure, and thence into the activesemiconductor structure.

The selection of the composition and ordering of the layers of the frontelectrical contact is critical and is not arbitrary. Titanium adhereswell to semiconductor materials, particularly when gold or otherdiffusion metal is diffused into and through the titanium layer to thebondline region between the front electrical contact and the activesemiconductor structure. The titanium layer must therefore contact theactive semiconductor structure, and the diffusion layer must thereforeoverlie and contact the titanium layer so that gold or otherdiffusion-layer atoms may diffuse into the titanium layer duringfabrication and service. The joining layer must be the topmost layer ofthe front electrical contact, because it provides a good material forthe affixing of the front electrical lead to an attachment pad region ofthe front electrical contact.

However, the diffusion of metals commonly used in the joining layer andthe front electrical lead to the bondline region between the frontelectrical contact and the active semiconductor structure, and thenceinto the active semiconductor structure, may cause the degradation instructure and performance. The barrier layer must therefore lie betweenthe diffusion layer and the joining layer, so that it does not preventthe diffusion of the diffusion metal to the bondline region but doesprevent diffusion of the joining metal and the metal of the frontelectrical lead to the bondline region between the front electricalcontact and the active semiconductor structure, and thence into theactive semiconductor structure. As noted, the barrier layer cannot liebetween the active semiconductor structure and the titanium layer, orbetween the titanium layer and the diffusion layer, because barriermetals do not bond well to semiconductor materials and because itspresence in these locations would interfere with the bonding of thefront electrical contact to the semiconductor structure.

Comparative accelerated life test results of the present approachsuggest that a 3-4 fold increase in lifetime for this new front-contactstructure.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thescope of the invention is not, however, limited to this preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a solar cell from the front side;

FIG. 2 is an enlarged schematic sectional view on line 2-2 through thesolar cell of FIG. 1;

FIG. 3 is an enlarged schematic sectional view on line 3-3 through thesolar cell of FIG. 1; and

FIG. 4 is a block flow diagram of a preferred approach for fabricatingan embodiment of the present solar cell.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the pertinent features of a solar cell 20. The solarcell 20 includes an active semiconductor structure 22 having an activesemiconductor structure front side 24 and an active semiconductorstructure back side 26. A back electrical contact 28, illustrated as athin metallic layer, overlies and contacts the active semiconductorstructure back side 26. The back electrical contact 28 usually coversthe entire active semiconductor structure back side 26. A backelectrical lead 29 is affixed to the back electrical contact 28. Whenthe solar cell active semiconductor structure 22 is illuminated by thesun or other light source directed against the active semiconductorstructure front side 24, there is a voltage produced between the activesemiconductor structure front side 24 and the active semiconductorstructure back side 26. FIGS. 1-3 are not drawn to scale.

FIG. 2 shows a portion of this structure in sectional view, furtherillustrating the details of the active semiconductor structure 22. Thestructure and fabrication of such active semiconductor structures 22 areknown in the art, and only the basic structures are shown here. Theactive semiconductor structure 22 includes at least two, and in advancedsolar cells more than two, semiconductor layers 30. In the illustration,there are four semiconductor layers 30 a, 30 b, 30 c, and 30 d. Pairs ofthe semiconductor layers 30 define semiconductor junctions 32. In theillustration, semiconductor layers 30 a and 30 b form semiconductorjunction 32 a; semiconductor layers 30 b and 30 c form semiconductorjunction 32 b; and semiconductor layers 30 c and 30 d form semiconductorjunction 32 c. In practical cases, there are additional layers at thesemiconductor junctions 32 to facilitate their operation but are notshown here to avoid clutter in this schematic illustration. Whetherthere is a single semiconductor junction 32 or multiple semiconductorjunctions 32, the facing pairs of semiconductor layers 30 and theirrespective semiconductor junctions 32 are selected to respond to thewavelength components of the illuminating light to create a voltageacross the contacting semiconductor layers 30 and semiconductorjunctions 32. The selections of the specific compositions of thesemiconductor layers 30 of the solar cell active semiconductor structure22, and possible additional layers within the active semiconductorstructure, are not a part of the present approach. Typically, however,current technology uses doped silicon semiconductor layers 30 and/ordoped gallium arsenide semiconductor layers 30, and modifiedcompositions based upon these base compositions. The present approachmay be used with known or future active semiconductor structures 22.

A front electrical contact 34 overlies and contacts the activesemiconductor structure front side 24. A portion of the front electricalcontact 34 is a current collector 36, illustrated in the form ofmetallic strips 38, that contact the active semiconductor structurefront side 24 and, in cooperation with the back electrical contact 28,collect the current produced by the active semiconductor structure 22.These strips 38 do not cover the entire active semiconductor structurefront side 24, because the illumination must reach at least a portion ofthe active semiconductor structure front side 24. Another portion of thefront electrical contact 34 is a busbar 39 to which the currentcollected in the current collector 36 is conducted. The front electricalcontact 34 also includes an attachment pad region 40 to which a frontelectrical lead 52 is attached.

As shown in greater detail in FIG. 3, the front electrical contact 34includes a titanium layer 42 overlying and contacting the activesemiconductor structure front side 24, a diffusion layer 44 overlyingand contacting the titanium layer 42, a barrier layer 46 overlying andcontacting the diffusion layer 44, and a joining layer 48 overlying andcontacting the barrier layer 46. The titanium layer 42 is preferablypure titanium, but titanium alloys may be used. The diffusion layer 44is preferably gold, but other metals such as palladium may be used insome cases. The barrier layer 46 is preferably platinum, but nickel,palladium, rhodium, or other noble metals may be used. The joining layer48 is preferably silver, but other metals such as gold, aluminum, orcopper may be used. As used here unless otherwise stated, thespecification of a metal can include the unalloyed metal or alloys ofthe metal. In most cases, the pure metals are used to maximizeelectrical. conductivity, but in some cases alloys of the specifiedmetal with other elements may be used for improved strength or otherproperties.

The selection of the composition and ordering of the layers of the frontelectrical contact 34 is critical. Titanium adheres well tosemiconductor materials, particularly when gold is diffused into andthrough the titanium layer 42 to the bondline region 50 between thefront electrical contact 34 and the active semiconductor structure 22.The titanium layer 42 must therefore contact the active semiconductorstructure 22, and the diffusion layer 44 must therefore overlie andcontact the titanium layer 42 so that atoms from the diffusion layer maydiffuse into the titanium layer 44 and to the bondline 50 duringfabrication and service. The joining layer 48 must be the topmost layerof the front electrical contact 34, because it provides a good materialfor the affixing of a front electrical lead 52 to the front electricalcontact. The silver or other metal from the joining layer 48 and thematerial of construction of the front electrical lead 52 are potentiallydeleterious to the adhesion of the front electrical contact 34 to theactive semiconductor structure 22 and are also deleterious to theperformance of the active semiconductor structure 22. The barrier layer46 must therefore be placed between the joining layer 48 and thediffusion layer 44 to inhibit mass diffusion from the joining layer 48and the front electrical lead 52 to the bondline region 50 and into theactive semiconductor structure 22, but not inhibit diffusion of atomsfrom the diffusion layer 44 into the titanium layer 42.

The front electrical lead 52 may be a discrete wire, as illustrated,bonded, soldered, or welded to the attachment pad region 40 of the frontelectrical contact 34. It may instead be a layer deposited upon theactive semiconductor structure, or any other operable structure.

Preferably, the titanium layer 42 has a thickness of from about 50Angstroms to about 250 Angstroms, the diffusion layer 44 has a thicknessof from about 100 Angstroms to about 600 Angstroms, the barrier layer 46has a thickness of from about 100 Angstroms to about 1000 Angstroms, andthe joining layer 48 has a thickness of from about 20,000 Angstroms toabout 70,000 Angstroms. The titanium layer 42 must be sufficiently thickto achieve full coverage over the active semiconductor structure frontside 24, but not so thick that it prevents diffusion of the metal fromthe diffusion layer 44 to the active semiconductor structure front side24. The diffusion layer 44 must be sufficiently thick to ensure adhesionof the front electrical contact 28 to the active semiconductor structurefront side 24, but not so thick that it degrades the adhesion by othermechanisms such as thermal fatigue due to differences in thermalexpansion coefficients. The barrier layer 46 must have a sufficientlygreat thickness to achieve the required diffusion barrier, but not sothick that it degrades the adhesion by other mechanisms such as thermalfatigue due to differences in thermal expansion coefficients. Thethickness of the joining layer 48 is selected for convenience.

FIG. 4 depicts a preferred approach for fabricating a preferred solarcell by the present approach. The active semiconductor structure 22 andthe back-side structure, including the back electrical contact 28, areprovided, numeral 60. Typically, the semiconductor layers 30 of theactive semiconductor structure 22 are deposited upon a substrate,numeral 62, and the back electrical contact 28 is deposited overlyingand contacting the active semiconductor structure 22, numeral 64.Techniques and processes for performing the step 60 and its substeps areknown in the art, see for example U.S. Pat. No. 5,330,585, whosedisclosure is incorporated by reference.

The front electrical contact 34 is applied, numeral 66, overlying andcontacting the active semiconductor structure front side 24, bysequentially depositing the layers that form the front electricalcontact 34. The titanium layer 42 is first deposited overlying andcontacting the active semiconductor structure front side 24, numeral 68.The lateral extent of the titanium layer 42 defines the front electricalcontact 34. The titanium layer 42 may be deposited using a stand-offmask or a photolithographic mask. The diffusion layer 44 is thereafterdeposited overlying and contacting the titanium layer 42, numeral 70.The barrier layer 46 is thereafter deposited overlying and contactingthe diffusion layer 44, numeral 72. The joining layer 48 is thereafterdeposited overlying and contacting the barrier layer 46, numeral 74.

The sequential application of the various layers 42, 44, 46, and 48 ofthe front electrical contact 34, numeral 66, may be performed by anyoperable approach, but is preferably performed by vacuum vapordeposition, a known technique for other applications. In vacuum vapordeposition, the structure that is to be deposited upon (here thealready-fabricated active semiconductor structure 22) is loaded into avacuum chamber. The vacuum chamber is pumped down to a low depositionpressure, and the deposition is performed. Each pumpdown of the vacuumchamber from ambient pressure to the deposition pressure is timeconsuming and therefore an expensive step in the fabrication of thesolar cells. One of the desirable features of the preferred approach isthat the entire front electrical contact 34 may be deposited by vacuumdeposition in a single pumpdown or evacuation of the vacuum chamber fromambient pressure. The layers 42, 44, 46, and 48 are sequentiallydeposited from different deposition sources within the same vacuumchamber during the single pumpdown cycle. Steps 64 and 66 may bereversed in order, with the front electrical contact 34 deposited beforethe back electrical contact 28 is deposited.

Lastly, the front electrical lead 52 and the back electrical lead 29 areaffixed to their respective locations, numeral 76. The affixing may beaccomplished by any operable technique, such as wire bonding, soldering,or welding. If one or both of the electrical leads 52, 29 are depositedlayers, they are deposited by any operable approach, such as vacuumvapor deposition.

Other features of the present approach, as discussed elsewhere herein,may be used in conjunction with this processing approach.

The preferred structure as depicted in FIGS. 1-3 has been practiced on alaboratory scale using the fabrication approach of FIG. 4. Solar cellshave been successfully fabricated. For comparative testing, thediffusion layer 44 was gold, the barrier layer 46 was platinum, and thejoining layer 48 was silver. Comparative accelerated life testing wasperformed against a prior art solar cell in which the front electricalcontact 34 was formed of a layer of titanium overlying the front side ofthe active semiconductor structure, a layer of gold overlying thetitanium layer, and a layer of silver overlying the diffusion layer.There was no barrier layer present in the comparison structure. Theaccelerated life testing was performed by holding the test solar cellsat a temperature of 270° C. in a nitrogen environment.

Degradation of the electrical performance of the solar cells madewithout the barrier layer was 3-4 times as fast as the degradationmeasured for the solar cells made by the present approach, with thebarrier layer 46 present.

Although a particular embodiment of the invention has been described indetail for purposes of illustration, various modifications andenhancements may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not to be limited except asby the appended claims.

1. A solar cell, comprising: an active semiconductor structure having anactive semiconductor structure front side and an active semiconductorstructure back side, wherein the active semiconductor structure producesa voltage between the active semiconductor structure front side and theactive semiconductor structure back side when illuminated from theactive semiconductor structure front side; a back electrical contactoverlying and contacting the active semiconductor structure back side; afront electrical contact overlying and contacting the activesemiconductor structure front side, wherein the front electrical contacthas multiple layers comprising: a titanium layer overlying andcontacting the active semiconductor structure front side, a diffusionlayer overlying and contacting the titanium layer, a barrier layeroverlying and contacting the diffusion layer, and a joining layeroverlying and contacting the barrier layer.
 2. The solar cell of claim1, wherein the solar cell further includes a front electrical leadoverlying and affixed to an attachment pad region of the frontelectrical contact.
 3. The solar cell of claim 1, wherein the frontelectrical contact comprises a current collector.
 4. The solar cell ofclaim 1, wherein the front electrical contact comprises a busbar.
 5. Thesolar cell of claim 1, wherein the diffusion layer is gold.
 6. The solarcell of claim 1, wherein the barrier layer is made of a barrier-layermetal selected from the group consisting of platinum, palladium,rhodium, and nickel.
 7. The solar cell of claim 1, wherein the joininglayer is made of a joining-layer metal selected from the groupconsisting of silver, gold, aluminum, and copper.
 8. The solar cell ofclaim 1, wherein the titanium layer has a thickness of from about 50Angstroms to about 250 Angstroms, the diffusion layer has a thickness offrom about 100 Angstroms to about 600 Angstroms, the barrier layer has athickness of from about 100 Angstroms to about 1000 Angstroms, and thejoining layer has a thickness of from about 20,000 Angstroms to about70,000 Angstroms.
 9. The solar cell of claim 1, wherein the solar cellactive semiconductor structure comprises a doped silicon layer or adoped gallium arsenide layer.
 10. A method for fabricating a solar cell,comprising the steps of providing an active semiconductor structurehaving an active semiconductor structure front side and an activesemiconductor structure back side, wherein the active semiconductorstructure produces a voltage between the active semiconductor structurefront side and the active semiconductor structure back side whenilluminated, and a back electrical contact overlying and contacting theactive semiconductor structure back side; applying a front electricalcontact overlying and contacting the active semiconductor structurefront side, wherein the front electrical contact has multiple layerscomprising: a titanium layer overlying and contacting the activesemiconductor structure front side, a diffusion layer overlying andcontacting the titanium layer, a barrier layer overlying and contactingthe diffusion layer, and a joining layer overlying and contacting thebarrier layer.
 11. The method of claim 10, wherein the step of applyingincludes the step of sequentially depositing the titanium layer, thediffusion layer, the barrier layer, and the joining layer.
 12. Themethod of claim 10, wherein the step of applying includes the step ofsequentially vacuum depositing the titanium layer, the diffusion layer,the barrier layer, and the joining layer in a vacuum depositionapparatus in a single pumpdown from ambient pressure.
 13. The method ofclaim 10, including an additional step of affixing a front electricallead overlying and contacting an attachment pad region of the frontelectrical contact.
 14. The method of claim 10, wherein the step ofapplying includes the steps of applying the titanium layer to athickness of from about 50 Angstroms to about 250 Angstroms, applyingthe diffusion layer to a thickness of from about 100 Angstroms to about600 Angstroms, applying the barrier layer to a thickness of from about100 Angstroms to about 1000 Angstroms, and applying the joining layer toa thickness of from about 20,000 Angstroms to about 70,000 Angstroms.15. The method of claim 10, wherein the step of applying includes thestep of applying platinum, palladium, rhodium, or nickel as the barrierlayer.
 16. The method of claim 10, wherein the active semiconductorstructure comprises a doped silicon layer or a doped gallium arsenidelayer.
 17. A method for fabricating a solar cell, comprising the stepsof providing an active semiconductor structure having an activesemiconductor structure front side and an active semiconductor structureback side, wherein the active semiconductor structure produces a voltagebetween the active semiconductor structure front side and the activesemiconductor structure back side when illuminated, and a backelectrical contact overlying and contacting the active semiconductorstructure back side; applying a front electrical contact overlying andcontacting the active semiconductor structure front side, wherein frontelectrical contact has multiple layers comprising: a titanium layeroverlying and contacting the active semiconductor structure front side,a gold layer overlying and contacting the titanium layer, a platinumlayer overlying and contacting the gold layer, and a silver layeroverlying and contacting the platinum layer, wherein the step ofapplying includes the step of sequentially vacuum depositing thetitanium layer, the gold layer, the platinum layer, and the silver layerin a vacuum deposition apparatus in a single pumpdown from ambientpressure; and affixing a front electrical lead overlying and contactingan attachment pad region of the front electrical contact.